The invention described herein may be manufactured, used, and/or licensed by or for the United States Government.
1. Field of the Invention
The present invention relates generally to onium salts and to methods of producing the same. These salts, obtained by the methods provided herein, are useful components in a wide spectrum of fields, such as phase transfer catalysis, electrolyte solutes for aqueous or non-aqueous electrochemical devices, various additives, and medicaments, etc. More particularly, the present invention provides a new family of asymmetric onium cations, which when combined with an appropriate anion, result in salts having high electrochemical stability and high solubility in non-aqueous polar solvents. Most particularly, the present invention relates to the formulation of a non-aqueous electrochemically stable electrolyte solution comprising these onium salts and an appropriate solvent or solvent mixture. Finally, the present invention relates to improved electrochemical capacitors utilizing these novel electrolyte solutions and thereby having improved energy density and power capabilities.
2. Description of the Related Art
Electrochemical capacitors are energy storage devices that are able to store and release energy by the means of ion adsorption/desorption on high surface area electrodes. These capacitors typically consist of two porous electrodes that are isolated from electrical contact with each other by a separator. Both the separator and the electrodes are impregnated with an electrolytic solution, i.e., a salt or mixture of salts dissolved in appropriate solvent or mixture of solvents.
When electric potential is applied across the two electrodes during charge, ionic current flows within the capacitor due to the attraction of anions by the positive electrode and cations by the negative electrode. Upon reaching the surface of their respective electrodes, equal amounts of anions and cations are absorbed in the electrode/electrolyte interphase and are held in the region by the opposite charges in the solid electrode.
The above state of charge-separation tends to go back to the ground level of lower energy where no charge is separated. Thus, when the two electrodes are no longer held at separate potential and are connected via a load, these absorbed cations and anions desorb from the electrode/electrolyte interphase and migrate back to the bulk of the electrolyte. During this process the current produced within the capacitor drives the load as the capacitor is discharged. The above process can be repeated tens of thousands of times.
The rate at which energy can be stored/released in such capacitors is extremely high, on the order of 500xcx9c3000 W/Kg, which is higher than most electrochemical energy devices including the state-of-the-art Li-ion batteries (50xcx9c300 W/Kg). However, a disadvantage for capacitors is their low to moderate energy densities, 5xcx9c10 Wh/Kg compared to 40xcx9c200 Wh/Kg for Li-ion batteries.
The energy output (stored energy) of such capacitors is described by the following formula:                     E        =                              1            2                    ⁢                      xe2x80x83                    ⁢          C          ⁢                      xe2x80x83                    ⁢                                    (                              Δ                ⁢                                  xe2x80x83                                ⁢                V                            )                        2                                              (        1        )            
Where E is the storable energy at a potential difference xcex94V between the electrodes and C is the storage capacitance of the electrodes (B. E. Conway, J. Electrochem. Soc., 1991, 138, 1539). For a given electrode material with a certain C, it is desirable to increase the operating potential xcex94V in order to obtain high energy density output. However, this operating potential is always restricted by the stability limit of the solvent and salt.
For aqueous or any protic solvents, the stability limit imposed by the reduction of proton and/or oxidation of hydroxyl ion is ca. 1.2xcx9c2.0 V. Earlier efforts aimed at increased operating potential led to the use of non-aqueous aprotic solvents. See, for example, Boos et al., U.S. Pat. No. 3,536,963, and Yoshida et al., U.S. Pat. No. 5,150,283, describing electrolytes of non-aqueous solvents and ammonium salts, among others, which usually can support up to 3.0 V potential difference.
For these electrolyte solutions the stability limit is usually imposed by the decomposition of the salts, especially at the negative potential extreme, where the cation usually determines the cathodic stability limit of the electrolyte alone, independent of the anion and the solvent it is in.
Therefore it is highly desirable to find a new electrochemically stable salt, which, when dissolved in non-aqueous aprotic solvents, can provide high resistance toward oxidation and reduction. More specifically, the salt should have a cation which is stable against reduction at the negative electrode, and an anion which is stable against oxidation at positive electrode, and their stability should be higher or at least as high as that of the solvent. Thus, any improvement in electrochemical stability will increase operating potential (xcex94V) and have an impact on energy output by the magnitude squared as shown by Equation (1).
Furthermore, energy density can be affected by the number of ions available in the electrolytic solution (J. P. Zheng, J. Huang, T. R. Jow, J. Electrochem. Soc., 1997, 144, 2026). In other words, limited solubility of most salts in aprotic, non-aqueous solvents often limits the energy density at high operating voltages. At high rate discharge/charge operations, the number of ions available in the electrolytic solution also limits the power output, i.e., where high demand for ions lowers the ion concentration in the solution thus increasing the resistance and limiting the power output. It is therefore also highly desirable to find a salt having higher solubility in aprotic, non-aqueous solvents.
The electrolyte used in state-of-the-art electrochemical capacitors contains tetraethylammonium tetrafluoroborate (Et4NBF4) in propylene carbonate (PC) solvent as described in U.S. Pat. No. 5,150,283, A. Yoshida and K. Imoto, xe2x80x9cElectric Double Layer Capacitor and Method for Producing the Samexe2x80x9d; or the same salt in acetonitrile (AN) solvent as described in U.S. Pat. No. 5,621,607, C. J. Farahmandi and J. M. Dispennette, xe2x80x9cHigh Performance Double Layer Capacitors Including Aluminum Carbon Composite Electrodesxe2x80x9d. However, these electrolytes have serious shortcomings.
For example, the electrolyte solution of Et4NBF4 in PC exhibits low salt solubility, having a saturated concentration of 0.86 M at room temperature, and low conductivity of 8.8 mS/cm at 0.65 M at room temperature. This electrolyte is suitable for low power applications such as memory protection but not for high power applications.
In contrast, the electrolyte solution of Et4NBF4 in AN has high conductivity, about 50 mS/cm at 1.4 M at room temperature, and the saturated salt concentration is about 1.68 M at room temperatures. However, the operating voltages of the capacitor using this electrolyte is about 0.5 V lower than that using the electrolyte of Et4NBF4 in PC. Furthermore, the high vapor pressures of AN makes it unsuitable for applications at elevated temperatures.
Where both high salt solubility and high operational voltage are desired for an electrolyte, the state-of-the-art solutes comprising symmetrical quaternary ammonium salts such as tetraethyl ammonium salt are inadequate. The present invention fulfills these needs by providing asymmetrical onium salts or mixtures of such salts in aprotic, non-aqueous solvents or mixtures of such solvents. These novel electrolytes are able to perform at a high rate of charge/discharge, at low operating temperatures, and within a wide range of operating voltage due to the high solubility, low melting temperature, and the improved reduction stability of the new onium cations, respectively.
Accordingly, it is a primary object of the present invention to provide an electrochemically stable salt.
It is another object of the present invention to provide an electrochemically stable salt having electrochemically stable cations against reduction at the negative electrode, and electrochemically stable anions against oxidation at the positive electrode.
It is still another object of the present invention to provide a salt which comprises electrochemically stable onium cations, and electrochemically stable inorganic or organic anions.
It is still another object of the present invention to provide an electrochemically stable salt which also has high solubility in an aprotic, non-aqueous solvent or mixtures of such solvents.
It is still another object of the present invention to provide an electrochemically stable salt having a low melting temperature.
It is yet another object of the present invention to provide an electrolyte formulation comprising an electrochemically stable onium salt or mixture of such salts dissolved in an aprotic, non-aqueous solvent or mixture of such solvents.
It is a still further object of the present invention to provide an electrolyte capable of performing at a high rate of charge/discharge, at low ambient temperatures, and within a wide range of operating voltages.
It is a still further object of the present invention to provide an electrochemical capacitor comprising two porous electrodes, a separator, and the aforementioned electrolyte having an electrochemically stable salt solute.
In satisfaction of the foregoing objects and advantages, the present invention provides a novel family of onium salts which, in appropriate solvents, will form electrolyte solutions having these desirable properties.